The real reason for everything being in your center comes from the 2 handed Katana. Center cuts were the strongest.

I think you are putting the cart before the horse -- without two hands (and the rest of the bipedal human body structure) to grasp the katana, there would be no centerline for your hands to occupy.

However, even a radially designed creature like the octopus understands the principals of "front toward enemy" and "centerline(s) is where you can most effectively apply energy from both/all sides of your body.

I propose that (yet another viewpoint) your 'center' can be thought of as the intersection of all the centerlines created by your ability to effectively strike an opponent with any pair of limbs or extremities you possess, whether forwards, to the rear, above, to the ground, or to the sides. As such, the center is not a fixed point within your body under all conditions, not an absolute, but can be thought of as a "strange attractor" bound by the equations of your limbs -- the location of your center is relative to your posture and the position of your limbs at any moment in time. It may be bound within a limited region, but it cannot be fixed as an absolute to any one point.